re ZON-3253: Remove schema field that is irrelevant.
re ZON-3253: Remove schema field that is irrelevant.

- The schema for additional fields comes from the reference object defined in
  ``zeit.content.volume.interfaces.IVolumeReference``.

ZeitOnline/zeit.content.article,ZeitOnline/zeit.content.article,ZeitOnline/zeit.content.article